Canine cyclic hematopoiesis (CH) is now known to be of marrow origin and the humoral regulators, erythropoietin and colony stimulating factor, cycle in a manner compatible with peripheral reticulocyte and leukocyte values. A colony of dogs with this genetic defect will be maintained and used to study CH bone marrow cell growth and differentiation under a variety of culture conditions. Marrow cells aspirated at all phases of the cycle will be cultured in the granulocytic soft agar system and the erythroid plasma clot system. Aspirated cells from each day in the CH cycle will be cryo-preserved and ultimately returned, cultured in Millipore diffusion chambers, to the autologous hosts at each phase of the hosts' humoral regulatory cycles. A different CH dog will be used for each autologous host study. Further, large volume aspirates of the CH marrow will be separated into various cell fractions and cultured in the foregoing systems. An accurate assessment of cell growth-differentiation of marrow aspirate, and fractions of aspirates, cultured in vitro under CSF and ESF stimulation and in the CH dog with cyclic ESF-CSF, will determine if CH marrow replicates in a cyclic manner independent of humoral stimuli or is dependent on interrelated responses to humoral stimuli.